Helicopters include one or more main rotors each having a plurality of rotor blades. The rotor blades are rotatably driven by a drive mechanism such as an engine via a transmission. Helicopter flight is regulated by cyclic and collective control of the angle of incidence or pitch angle of the rotor blades.
In collective control, the pitch angle of the rotor blades is collectively changed all at once such that the amount of lift produced by the rotor blades is altered by the same amount. Collective control is used to change the helicopter's altitude and/or airspeed. In cyclic control, the individual pitch angle of each rotor blade is changed or cycled as the rotor blade rotates such that different amounts of lift are produced at different times during a revolution. Cyclic control is used to change the helicopter's sideways direction (i.e., roll) and forward/aft direction (i.e., pitch).
In prior art helicopters, cyclic and collective control inputs may be generated by pilot command and are provided to the rotor blades through a rotating swashplate. The swashplate is connected to each individual rotor blade through a set of pitch links and pitch horns. The swashplate transmits the pilot's commands from the non-rotating frame of the helicopter (i.e., the fuselage) to the rotating frame (i.e., the rotor hub and rotor blades).
One of the primary problems associated with helicopter flight is excessive amounts of vibration and noise generated by rotation of the main rotor blades. The vibration is transmitted from the rotor hub through the helicopter transmission to produce vibration in the fuselage. Rotor-induced vibration may occur as a result of rotor-induced shear forces and moments acting at the rotor hub and can reduce passenger and crew comfort and cause fatigue damage to structural components of the helicopter. The majority of vibration produced by the rotor blades is transmitted to the rotor hub at an excitation frequency which is an integral multiple of the number of rotor blades in the main rotor. For an n-bladed main rotor, the predominant excitation or vibration frequency at the rotor hub is the “nth” harmonic. For example, in a three-bladed main rotor, the predominant vibration frequency is the third harmonic of the rotary frequency of the main rotor.
Rotor-induced noise is typically comprised of a combination of aerodynamic loading noise and turbulence noise and is a function of rotor blade lift and the rotational speed of the main rotor. Rotor-induced noise may also be generated by various aerodynamic phenomena such as blade-vortex-interaction (BVI) wherein one rotor blade passes through the shed tip vortices of a previous rotor blade. Rotor-induced noise can also reduce passenger and crew comfort and is frequently the source of complaints from individuals due to helicopter overflights.
Rotor-induced noise and vibration are also associated with aerodynamic characteristics of a helicopter. Flight performance and efficiency can be limited by the ability to control the pitch angle of the rotor blades. For example, forward speed of the helicopter may be limited by inefficiencies associated with non-optimal pitch angles of the rotor blades at any given point during the revolution of each rotor blade. In order to overcome such performance limitations and to reduce noise and vibration, it is desirable to modulate the pitch angle of each rotor blade as it rotates through its azimuth.
Efforts have been directed toward development of several active control systems for minimizing the magnitude of rotor-induced noise and vibration and to improve aerodynamic performance and efficiency. Such active control systems include high harmonic control (HHC) and individual blade control (IBC). In HHC, high frequency pitch angle changes are induced in the rotor blades to counteract the vibration frequencies produced by the rotor blades.
Unfortunately, due to geometry and configuration constraints of conventional swashplate designs, only certain frequencies (i.e., the nth harmonic of an n-bladed rotor and its immediately adjacent frequencies) can be transmitted to the rotor blades. For example, in a three-bladed rotor, only the third harmonic (i.e., the n-blade harmonic) and the second and forth harmonics (i.e., the immediately adjacent harmonics) can be transmitted to the rotor blades. Other inherent drawbacks such as backlash and friction in the mechanical linkages between the swashplate and the rotor blades limits the overall ability of HHC to counteract vibration.
IBC is another active control system that allows for independent pitch angle modulation of each rotor blade in addition to the pitch angle control inputs provided by a conventional swashplate. In contrast to HHC, IBC provides the ability to counteract a variety of different vibration frequencies other than the predominant frequency (i.e., the nth harmonic of an n-bladed rotor) and its immediately adjacent frequencies. In addition, IBC permits pitch angle adjustments in a variety of signal forms and is not limited to the sinusoidal waveform inputs of conventional swashplate configurations.
One attempt at implementing an IBC actuation system in order to overcome noise and vibration problems is an electromechanical/hydraulic arrangement including a set of electromechanical actuators (EMA's) coupled to the rotor blades. In one embodiment, the EMA's are vertically-oriented in parallel relationship to the rotor shaft and are configured to provide primary flight control inputs (i.e., directional and lift) as well as individual control inputs to adjust the rotor blade pitch angles.
Unfortunately, because each EMA includes only a single, synchronous motor, the ability of the EMA to overcome certain types of failures may be limited. For example, in a “hardover” failure of any one of the motors, the pitch angle of any one blade moves to its extreme position which may compromise the reliability of the aircraft.
A further drawback associated with the above-described EMA configuration is the relative complexity of the electromechanical and hydraulic actuator system. In addition, the EMA configuration is understood to be relatively heavy, bulky and requiring a large amount of maintenance and ground support equipment for servicing and maintaining the multiple hydraulic pumps, hydraulic modules, reservoirs and tubing.
As can be seen, there exists a need in the art for a system and method for individual blade control to reduce helicopter vibration and noise. In addition, there exists a need in the art for a system and method for individual blade control that improves aerodynamic performance and economy and allows for greater flexibility in rotor blade control as compared to conventional swashplate configurations. Finally, there exists a need in the art for a system and method for individual blade control that is highly reliable, light weight, simple in construction and which requires minimal maintenance.